The Nintendo Switch does not have an official "UI download link" because its system interface is built into the firmware and updated automatically through the console's settings.
There is for consumers. Your Switch downloads updates automatically when connected to the internet. To manually force an update: nintendo switch ui download link
While there isn't an official "download link" for the Nintendo Switch user interface (as it’s proprietary system software built into the hardware), the internet is full of "reviews" for fan-made replicas, PC themes, and custom firmware skins. The Nintendo Switch does not have an official